The present invention relates to an electrophotographic image forming apparatus, and more particularly relates to an image forming apparatus which forms an electrostatic latent image on a photoreceptor by light exposure which has been beam-modulated according to digital image data sent from a computer.
Recently, in the field of electrophotography in which an electrostatic latent image is formed on a photoreceptor and the latent image is developed so that a visual image can be obtained, a digital system of image forming has been actively investigated by which improvement, conversion and editing of images are easily conducted so that image forming of high quality is possible.
By the aforementioned image forming method, each dot of an image is formed as follows: a dot-shaped exposure is conducted on a photoreceptor which has been uniformly charged, by light emitting elements such as a laser, a LED array, a liquid crystal shutter and the like, according to digital image signals sent from a computer or a copy document.
Concerning an optical scanning system in which optical modulation is conducted according to digital image signals, an apparatus has been proposed in which a semiconductor laser is utilized and a laser beam is directly modulated by the method of pulse-width modulation (Japanese Patent Application Open to Public Inspection 39976/1987).
The section of a beam which has been modulated by a digital image signal is circular or oval, and its luminance distribution is similar to a normal distribution, both foot ends of which are spread to the right and left. For example, in the case of a semiconductor laser beam, the luminance is usually 1-6 mW, and its sectional shape on a photoreceptor is a very narrow circle or oval of which one or both of the primary and subsidiary scanning lengths is 20-100 .mu.m.
However, even when an electrostatic latent image formed by the aforementioned beam is preferably developed by the method of reversal development, the sharpness of the obtained image is low in many cases.
In the case of an apparatus in which multi-value modulation is conducted by the method of direct intensity modulation or pulse modulation, the change of beam intensity is small with regard to an initial driving current of a laser semiconductor, and the change tends to fluctuate due to environmental factors such as heat. Further, the linearity with regard to a drive current is not good and there is a problem in the response to a drive signal, so that it is necessary to compensate by improving its electrical circuit. Therefore, when intensity modulation or pulse-width modulation is used for a multi-value modulation method, it is difficult to conduct the intensity modulation or pulse-width modulation faster. Especially, in the case of the intensity modulation, the linearity is so inferior that it is difficult to put it to practical use.
The aforementioned problems are ascribed to: the sensitivity of a photoreceptor is generally high in the beginning of exposure, so that the fluctuation of a beam tends to be picked up and a sharp dot-shaped latent image can not be formed.
FIG. 2a is a schematic illustration which shows a characteristic of a low .gamma. type of photoreceptor.
A low .gamma. type of photoreceptor in which light decay is sharp in the beginning of exposure and gentle in the middle of exposure as shown in FIG. 2a, has been used as a photoreceptor which is applied to an electrophotographic image forming apparatus.
Concerning the low .gamma. type of photoreceptor, the following have been widely known: a mono-layer type such as Se, CdS and the like; and a two-layer type composed of an electric charge generating layer and an electric charge conveyance layer, the two-layer type being used in usual Organic Photo Conductor. The light sensitivity of many of the photoreceptors which show the aforementioned semiconductor characteristic, is generally low in a low electric field, compared with a high electric field, and when the electric potential is lowered due to an increase in the amount of light, the sensitivity is lowered. For the reason described above, this type of photoreceptor is exclusively used in analog copiers in order to reproduce the gradation.
If an electrostatic latent image were formed on a low .gamma. photoreceptor by beams emitted from the aforementioned optical scanning system, then a sharp dot-shaped latent image could not be formed because the sensitivity of the aforementioned photoreceptor is generally high in the beginning of exposure, so that the fluctuation of beams tends to be picked up.
Even when an electrostatic latent image formed by the aforementioned beams is preferably developed by the method of reversal development, the sharpness of the obtained image is low in many cases.
In order to solve the problems explained above, the inventors have developed an image forming method and an image forming apparatus, the characteristic of which is described as follows. A dot-shaped electrostatic latent image is formed by a beam sent from an optical scanning system on a photoreceptor (which will be called a high .gamma. photoreceptor, hereinafter) having a light decay characteristic which is characterized in that: light decay does not occur in the beginning of exposure and light decay is sharp in the middle of exposure.
In the aforementioned image forming method and image forming apparatus, a high .gamma. photoreceptor is adopted, so that the surface potential of the photoreceptor is not lowered until the amount of light reaches a predetermined value. Accordingly, an electrostatic image is not formed in the right and left foot-end portions of the beam. As a result, a sharp dot-shaped electrostatic latent image can be formed.
FIG. 2b is a graph showing the outline of the light decay characteristic of a high .gamma. photoreceptor.
In the drawing, V.sub.1 represents a charging potential, V.sub.0 represents an initial potential at the beginning of exposure, L1 represents the light amount (.mu.J/cm.sup.2) of laser beam which is needed when initial potential V.sub.0 decays to 4/5, and L.sub.2 represents the light amount (.mu.J/cm.sup.2) of laser beam which is needed when initial potential V.sub.0 decays to 1/5.
A preferable range of the ratio L.sub.1 /L.sub.2 is as follows. EQU 1.0.ltoreq.L.sub.2 /L.sub.1 .ltoreq.1.5
In this embodiment, V.sub.1 =1000(V), V.sub.0 =950(V), and L.sub.2 /L.sub.1 =1.2. The potential of the photoreceptor at exposed portion is 10 V.
The light sensitivity in a position corresponding to the middle period of exposure in which initial potential (V.sub.0) is decayed to 1/2, is defined as E.sub.1/2, and the light sensitivity in a position corresponding to the initial period of exposure in which initial potential (V.sub.0) is decayed to 9/10, is defined as E.sub.9/10. Then, a photoconductive semiconductor giving the following relations is selected. EQU (E.sub.1/2)/(E.sub.9/10 .gtoreq.2
or preferably EQU (E.sub.1/2)/(E.sub.9/10).gtoreq.5
In this case, the photosensitivity is defined as an absolute value of potential lowering, with regard to a minute amount of exposure light.
As shown in FIG. 2b, the light decay curve of the photoreceptor 1 is characterized in that: the absolute value of a differential coefficient of the potential characteristics, which is light sensitivity, is small when the amount of light is small, and the absolute value is sharply decayed when the amount of light is increased. Specifically, as shown by the light decay curve in FIG. 2b, in the initial period of exposure, the sensitivity characteristic is low for a certain period L.sub.1 and shows a gentle light decay characteristic. However, in the middle of exposure from L.sub.1 to L.sub.2, the characteristic curve becomes highly sensitive and a super high .gamma. characteristic is obtained in which the characteristic curve descends almost linearly. It can be considered that: specifically, the photoreceptor 1 obtains a high .gamma. characteristic utilizing an avalanche phenomenon under the condition of a high potential charging of +500-+2000 V. In other words, it can be considered as follows: in the initial period of exposure, the carrier generated on the surface of a photoconductive pigment is effectively trapped by a surface phase between the aforementioned pigment and a coating resin, so that light decay can be positively inhibited; and as a result, a sharp avalanche phenomenon occurs in the middle period of exposure.
It is a primary object of the present invention to provide an image forming apparatus in which a high .gamma. photoreceptor is utilized, and which is characterized in that: the response to a drive signal is improved; and the variation of beam intensity which varies according to environmental factors such as heat, is reduced so that a latent image can be stably formed.
In the image forming method and apparatus in which a high .gamma. photoreceptor is used, it is important to adjust the diameter of a dot because the diameter of a dot-shaped latent image is brought about by modulating an image signal by the method of intensity modulation or pulse modulation in order to reproduce a middle tone.
However, compared with a low .gamma. photoreceptor, a high .gamma. photoreceptor needs a large amount of exposure light in order to reduce a surface potential after the photoreceptor has been uniformly charged. When dot exposure is conducted by a minute diameter of light, the amount of light exposure is so small that a latent image can not be formed. Namely, there is a problem in that: the diameter range of a dot-shaped electrostatic latent image which is formed by an optical scanning system, is limited, so that there is a defect when a middle tone image is reproduced.
It is a secondary object of the present invention to form an image in which a middle tone is accurately reproduced by a dot-shaped latent image.